The present invention relates in general to preloaded duplex ball bearings and in particular, to preloaded duplex ball bearings having a centrifugal debris trap.
Hard-preloaded duplex ball bearings of the type used to support high speed gas turbine engine main rotors are more vulnerable to failure than common single row thrust bearings. One reason is that hard-preloaded duplex ball bearings are more sensitive to contamination particles in lubricating oil. Small particles trapped between the rolling contact surfaces are pressed into the raceway with greater force than a single ball bearing produces, resulting in debris damage leading to premature rolling contact fatigue failure. The bearings also have an increased sensitivity to thermal gradients that can cause either an increase or decrease in operating thrust preload. These are the two factors responsible for the majority of failures of this type of bearing. The commonly used hard-preloaded duplex bearing is particularly sensitive to these factors due to the high axial stiffness inherent in the hard-preloaded configuration. Despite increased risk of failure, preloaded duplex ball bearings are utilized in applications where the axial thrust load may approach zero, or reverse.
U.S. Pat. No. 5,316,393 describes a preloaded duplex ball bearing having two springs for preloading. The spring preload is used to allow thrust load reversals and to prevent skid damage and provide robustness to thermal gradients. The preloaded duplex ball bearing does not, however, provide a means for preventing damage from contamination particles found in lubricating oil.
U.S. Pat. No. 6,179,933 describes a method of treating rolling element bearings for improved wear resistance. The raceway surfaces of the rolling element bearing are subjected to plasma nitriding to increase the hardness of the bearing components and to introduce compressive residual stress. The increased surface hardness and compressive residual stress gives improved resistance to debris damage and increases rolling contact fatigue life. While plasma nitriding the rolling element bearing helps against debris damage, it does not remove the contamination particles found in lubricating oil and with time, the hardened bearing will eventually fail.
As can be seen, there is a need for a preloaded duplex ball bearing that is less sensitive to thermal gradients as well as being able to withstand and even eliminate, contamination particles found in lubricating oil.